The present invention relates generally to high-k field effect transistors, and relates more particularly to flatband voltage tuning of high-k field effect transistors.
Flatband voltage is the voltage at which there is no electrical charge in a semiconductor and, therefore, no voltage drop across the semiconductor (i.e., in a band diagram the energy bands of the semiconductor are horizontal (flat)). Flatband voltages of high-k based p-type field effect transistors (pFETs) and n-type field effect transistors (nFETs) are difficult to control.
For instance, in conventional metal-oxide-semiconductor FETs (MOSFETs), the threshold voltage often fails to meet the desired value (e.g., ±0.3 V for both pFETs and nFETs). In some cases, electrode stacks are chosen according to workfunction to provide the desired flatbands, but this often does not work. Moreover, threshold voltage shifts can vary significantly from desired values, and the causes for such shifts, which may be numerous, are not fully understood.
Thus, there is a need in the art for a method and apparatus for flatband voltage tuning of high-k field effect transistors.